This invention relates to a method and/or apparatus for controlling the motor speed of an AC motor and/or a method of determining torque.
Squirrel cage induction motors are often desirable motors for industrial applications because of their low cost, ready availability, ruggedness and reliability. Until recently the speed of the motor has not been easily controllable but is determined by the number of poles, the mains supply frequency, and the motor loading. The motor loading causes small deviations in speed known as the motor slip.
Inverter drives for AC motors are now available at reasonable cost and allow the frequency of the voltage supplied to the motor to be varied over a wide range. The motor speed under no load conditions can be varied by adjusting the inverter output frequency, but small speed variations caused by changing motor loads are still present. To control the speed more accurately it is common practice to use a tacho-generator on the output shaft of the motor with a feedback control signal operating on the inverter output frequency.
Often it would be helpful to know what torque the motor is producing. Many techniques for doing this have been described but these usually involve controlling the flux in the motor's air-gap and then, operating with this constant flux, the torque can be determined. In many cases accurate shaft encoders must be used to determine the exact orientation of the motor's rotor so that the torque can be accurately calculated. Flux sensing and angle encoders are expensive and are not appropriate for a wide variety of applications.
To maintain constant air-gap flux, at full load, in a motor the motor voltage at low speeds must be increased, from the nominal constant volts/Hertz characteristic, to allow for the resistive losses in the stator winding of the machine. This voltage boosting is essential if rated torque is to be produced at low speeds, particularly at standstill, but it has the effect of increasing the motor losses under no load conditions. In a practical case the motor current in no-load conditions can be higher than the motor's full load current (at very low speeds) if the high voltage boost drives the motor into saturation. Thus if the voltage boost is high enough to give good starting performance it may well produce excessive motor losses on no load so that ultimately the motor will burn out even on no load. Summary of the Invention
It is an object of the present invention to provide a method and/or apparatus for controlling the motor speed of an electric AC motor and/or a method of determining torque which will go at least some distance toward overcoming the foregoing disadvantages or which will at least provide the public with a useful choice.
Accordingly, in one aspect the invention consists in a method of controlling the motor speed of an electric AC motor comprising the steps of:
(a) supplying a voltage having a frequency characteristic following substantially the relationship ##EQU1## where V.sub.1 =Terminal voltage PA1 M=Torque PA1 .omega.=supply frequency PA1 .omega..sub.s =rated slip PA1 (b) measuring the motor current, PA1 (c) multiplying the measured current signal and the supplied voltage, and PA1 (d) using the signal so formed to control the amplitude of the voltage supplied to the motor.
to said motor,
In a further aspect the invention consists in apparatus for controlling the motor speed of an AC electric motor comprising a current measuring means, a voltage source, multiplier means to multiply the sensed current, and voltage motor drive means, the output of said motor drive means comprising a voltage having a frequency characteristic following substantially the relationship ##EQU2## and having the output therefrom to the motor generated by the output from said multiplier means.
In a still further aspect the invention consists in a method of determining torque comprising the steps of adjusting an AC motor terminal voltage and current until the p.u. voltage and p.u. current at that frequency are equal, measuring the p.u. voltage or current and squaring the read voltage or current to give the motor torque.
To those skilled in the art to which the invention relates, many changes in construction and widely differing embodiments and applications of the invention will suggest themselves without departing from the scope of the invention as defined in the appended claims. The disclosures and the descriptions herein are purely illustrative and are not intended to be in any sense limiting.